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ISE 311Rolling labin conjunction withChapters 18 and 19 in the text book“Fundamentals of Modern Manufacturing”Third EditionMikell P. GrooverPrepared by: Amin Naser and Tom YelichMay 22nd, 2008

Outline

Introduction to rolling
Flat rolling and its analysis
Flat rolling defects
Rolling mills
Lab objective
Equipment
Lab Procedure
Summary

Introduction to Rolling

Rolling is a bulk deformation process in which the thickness of the

work is reduced by compressive forces exerted by two opposing

rolls. The rolls rotate to pull and simultaneously squeeze the work

between them.

The rolling process (specifically: flat rolling)

The basic process shown in the previous figure is “Flat Rolling”,

used to reduce the thickness of a rectangular cross section. A

closely related process is “shape rolling”, in which a square cross

section is formed into a shape such as an I-beam. (in this lab, you

will only do flat rolling)

Shape Rolling

Flat Rolling

Shape Rolling

Raw material and final product both in flat and shape rolling

After casting, ingots are rolled into one of three intermediate

shapes called blooms, billets, and slabs:

Blooms have square cross section 6” x 6” or larger. They are rolled into structural shapes.
Billets have square cross section 1.5” x 1.5” or larger. they are rolled into bars and rods.
Slabs have rectangular cross section 10” x 1.5” or larger. They are rolled into plates, sheets and strips.

As any other metal forming process, rolling can be performed hot (hot rolling) or cold (cold rolling).
Most rolling is carried out by hot rolling, owing to the large amount of deformation required.
Hot-rolled metal is generally free of residual stresses, and has isotropic properties. On the other hand, it does not have close dimentional tolerances, and the surface has a characteristic oxide scale. Moreover, cold rolled metals are stronger.

In this lab, you will only do cold rolling (at room temperature).

In flat rolling, the work is squeezed between two rolls so that its

thickness is reduced by an amount called the draft:

d = to - tf

where

d: draft

to: starting thickness

tf : final thickness

As a fraction of the starting thickness:

% reduction = % r = (d/ to)* 100%

Rolling increases work width. This is called “spreading”.
Spreading is expected because of the volume constancy in plastic deformation. Since the material is compressed in the thickness direction, both the length and width will increase provided that the material is not constrained in the width direction.
Spreading is more pronounced with low width-to-thickness ratios and low coefficients of friction, since there is small resistance to flow in the width direction.

The width-to-thickness ratio can be calculated as follows:

w/t Ratio = initial width/ initial thickness

After rolling, percentage spread can be calculated as follows:

% Spread = (Final width-initial width)/ (initial width) *100%

The work contacts the rolls along a contact arc. To calculate the arc length, you can use the following approximate formula:

L = (R * d)0.5

Where

L: approximate contact length, R: roll radius, d: draft

(See the next figure)

If the width-to-thickness ratio is large, then the spread will be negligible and a “plane-strain” condition may be assumed. Plane strain means that deformation occurs only in two directions (in a plane); the longitudinal (rolling) direction and the transverse direction.

This figure shows the contact

length between the work and

the rolls, the initial and final

work velocities, in addition to

the velocity of the rolls:

The work enters the gap between the rolls at a velocity vo and exits at a velocity vf. Because the volume flow rate is constant and the thickness is decreasing, vf should be larger than vo.
The roll surface velocity vr is larger than vo and smaller than vf. This means that slipping occurs between the work and the rolls.
Only at one point along the contact length, there is no slipping (relative motion) between the work and the roll. This point is called the “Neutral Point” or the “No Slip Point”.

The true strain experienced by the work in rolling is based on the

stock thickness before and after rolling:

ε = ln (to/tf)

The average flow stress in flat rolling can be can be determined

by:

Where:

K: Strength coefficient

n: Strain hardening exponent

(see the tensile testing module for more information about the power law σ = K εn)

In this experiment, a sheet will be rolled on 4 stages (4 passes).
In each pass, the average flow stress can be determined using the following formula:

where

Yavg, i : the average flow stress in the ith pass

: the strain after the i-1th (before the ith pass)= ln (to/ti-1)

: the strain after the ith pass = ε = ln (to/ti)

K : Strength coefficient

n : Strain hardening exponent

To calculate the roll force required to maintain separation

between the two rolls:

F = 1.15 * Yavg, i * Li * wi

where:

F : roll force

Yavg, i : the average flow stress in the ith pass

Li : the approximate contact length in the ith pass

wi : the width of the sheet in the ith pass

The torque in rolling can be estimated by:

T = 0.5 * F * L

Where:

T: Torque (lb.in or N.m)

F: Roll Force

L: Contact length

The Power required to drive the two rolls is calculated as follows:

P = 2π*N*F*L

Where:

P: Power (in J/s =Watt or in-lb/min)

N: Rolls rotational speed (RPM)

F: Roll Force

L: Contact length

From the previous equations we can conclude the following:

The contact length decreases by decreasing the roll radius.
The roll force depends on the contact length, and therefore, reducing the roll radius will reduce the roll force.
The torque and power depend on the roll force and contact length, and therefore, reducing the roll radius will reduce both the torque and power.
The power also depends on the rotational speed of the rolls, and therefore, reducing the rolls RPM will reduce the power.

On the entrance-side of the no slip point, the roll is faster than the sheet and therefore the friction force is in the rolling direction.
On the exit-side of the no slip point, the roll is slower than the sheet and therefore the friction force is opposite to the rolling direction.
The compression force on the rolls multiplied by the friction coefficient equals to the friction force.
Increasing the friction coefficient will shift the neutral point toward the entrance.

In rolling, friction force is important because it is responsible for pulling the sheet between the rolls.
Rolling may not be possible (the sheet will not be pulled) if the draft is large. The maximum draft for successful rolling is:

dmax = μ2 R

Where:

dmax : maximum draft for successful rolling

μ : coefficient of friction

R : roll radius

As can be seen from the equation, if μ is zero, then dmax is also zero (rolling is not possible)

Defects in rolling may be either surface or structural defects:

Surface defects include scale and roll marks.
Structural defects (see next figure) include:
Wavy edges:

bending of the rolls causes the sheet to be thinner at the edges, which

tend to elongate more. Since the edges are restricted by the material at the center, they tend to wrinkle and form wavy edges.

2. Center and edge cracks: caused by low material ductility and barreling of the edges.

3. Alligatoring: results from inhomogeneous deformation or defects in the original cast ingots.

Other defects may includes residual stresses (in some cases residual stresses are desirable).

Manufacturing Processes for Engineering Materials, 5th edition, S. Kalpakjian and S. Schmid

Structural defects in sheet rolling:

Wavy Edges Center cracking Edge cracking Alligatoring

Manufacturing Processes for Engineering Materials, 5th edition, S. Kalpakjian and S. Schmid

Rolling mills

Various rolling mill configurations are available (see next figure):

Two-high rolling mill: consists of two opposing rolls. These rolls may rotate only in one direction (nonreversing) or in two directions (reversing).
Three-high rolling mill: allows a series of reductions without the need to change the rotational direction of the rolls.
Four-high rolling mill:

Using small rolls reduces power consumption but increases the roll deflection. In this configuration, two small rolls, called working rolls, are used to reduce the power and another two, called backing rolls, are used to provide support to the working rolls.

4. Cluster rolling mill: another configuration that allows smaller working rolls to be used.

5. Tandem rolling mill: series of rolling stands .

Rolling mills

Various configurations of rolling mills:

Two-high

Three-high

Four-high

tandem

Cluster

Lab Objectives

This lab has the following objectives:

Introduce basic rolling parameters and some of the fundamental principles in rolling.
Calculate the roll forces required to reduce the thickness of a given aluminum strip.
Verify plane strain assumptions used in rolling analyses.
Identify where plane strain assumptions are not valid and to calculate the percentage spread in such cases.
Evaluate the strain hardening phenomena in rolling processes.
Identify some defects involved in rolling processes.

Equipment

In this lab, you will use a scale-down model of an industrial rolling mill. The rolling mill has two rolls powered by an electric motor.
The distance between the rolls, which is the roll gap, can be adjusted by rotating a pair of rotationally calibrated screws at the top of the roll stand (housing).
To keep the roll gap constant, you should ensure that the two adjusting screws have been adjusted to the same calibration number. There is a mark on each roll housing used to align the calibration marks on the adjusting screws.

Equipment

A picture of the scale-down rolling mill used in the lab:

Adjusting Screw

Rolls Housing/ Stand

Rolls

Equipment

Adjusting screw

Calibration numbers

Procedure

Part I: Rolling

1- Obtain the material data (K, n) from your lab instructor and record it in your datasheet.

2- Record the following initial conditions of your sample strip in the table provided:

Thickness
Length
Width
Hardness (HRC); average of three measurement

3- Set the roll gap for the first pass (set the adjusting screw to 5)

4- Start the rolling mill

Procedure

Part I: Rolling (continued)

5- Feed the strip through the mill (make sure not to feed the strip at an angle into the rolls).

6- In the table provided, record the:

Thickness
Length
Width
Hardness (HRC); average of three measurement

7- Reduce the roll gap by one unit per pass and repeat steps 3-6 for the

remaining four passes.

Procedure

Part II: Estimating the coefficient of friction

1- With a new sample, record the initial thickness.

2- Measure the roll radius.

3- Set the roll gap to 1 on the adjusting screw.

4- Start the rolling mill.

5- Very gently attempt to feed the strip through the mill, but do not force the strip into the mill. Hold the strip as level as possible and let friction between the rolls and the strip pull it in (this must be the case to get accurate data regarding the friction force). Note: the strip will not be pulled in on this first attempt.

Procedure

Part II: Estimating the coefficient of friction (Continued)

6- Open the roll gap by steps on one-half units on the roll set screw and repeat step 5 until the mill just pulls in the strip. The number on which the roll is set is not important and only the initial and final thickness of the specimen are needed.

7- Record the final thickness and complete the calculations with the

formulas provided.